1. Field of the Invention
The present invention generally relates to vaccines, and more particularly, to an HIV vaccine comprising immunogenic high-mannose type oligosaccharide clusters that mimics the HIV carbohydrate antigen having an affinity for the HIV-1 neutralizing antibody 2G12.
2. Background of the Related Art
HIV is a member of the lentivirus family of retroviruses. Retroviruses are small-enveloped viruses that contain a single-stranded RNA genome, and replicate via a DNA intermediate produced by a virally encoded reverse transcriptase, an RNA-dependent DNA polymerase.
The HIV viral particle comprises a viral core, composed in part of capsid proteins, together with the viral RNA genome and those enzymes required for early replicative events. A myristylated gag protein forms an outer shell around the viral core, which is, in turn, surrounded by a lipid membrane envelope derived from the infected cell membrane. The HIV envelope surface glycoproteins are synthesized as a single 160-kilodalton precursor protein, which is cleaved by a cellular protease during viral budding into two glycoproteins, gp41 and gp120. gp41 is a transmembrane glycoprotein and gp120 is an extracellular glycoprotein, which remains non-covalently associated with gp41, possibly in a trimeric or multimeric form.
Based on structural analysis, HIV-1 gp120 contains multiple high-mannose type N-glycans. These discontinuous oligosaccharide chains are grouped together to form a unique oligosaccharide microdomain. This high-mannose oligosaccharide grouping to form an epitope site has not been found in any human glycoproteins and is unique to HIV-1.
The worldwide epidemic of the human immuno-deficiency virus type 1 (HIV-1) urges the development of an effective HIV vaccine. Yet, it has been difficult to design effective immunogens that are able to elicit broadly neutralizing antibodies against HIV-1 primary isolates. In addition to sequence variability of neutralizing epitopes, HIV-1 has also evolved other mechanisms to evade immune attack, including change of conformations, shielding of conserved epitopes through heavy glycosylations, and formation of compact glycoprotein complexes (envelope spikes) that hinder the accessibility of epitopes to immune responses. It becomes clear that a successful strategy in developing an effective HIV-1 vaccine relies on the identification of conserved epitopes on HIV-1 that are accessible to neutralization and on the design of epitope-based immunogens that stimulate high immune responses.
So far, only a few human monoclonal antibodies (MAbs) have been identified that are able to neutralize a broad range of HIV-1 primary isolates. These include MAbs b12 and 2G12 that target the outer envelope glycoprotein gp120, and MAbs 2F5 and 4E10 that target the inner envelope glycoprotein gp41. The broadly neutralizing abilities of these MAbs implicate the existence of conserved and accessible antigenic determinants, i.e., epitopes, on the surface of most HIV-1 primary isolates. Passive immunization using these MAbs either alone or in combination has shown that these MAbs protect against HIV-1 challenge in animal models when present at sufficient concentrations prior to or shortly after exposure(12). However, results have been limited and determinative by concentration and ongoing re-immunization.
Among the broadly HIV-1 neutralizing antibodies so far identified, the human monoclonal antibody 2G12 is the only one that directly targets the surface carbohydrate antigen of HIV-1. Several pieces of evidence suggest that the epitope of 2G12 is a unique cluster of high-mannose type oligosaccharides (oilgomannose) on HIV-1 gp120. Initial mutational studies indicated that the oligomannose sugar chains at the N-glycosylation sites N295, N332, N339, N386, N392, and N448 might be involved in 2G12 recognition (9). Two recent studies further proposed that the epitope of 2G12 might consist of several Manα1-2Man-linked moieties contributed by the oligomannose sugar chains at sites N295, N332, and N392 that form a unique cluster on gp120 (81, 82).
However, HIV-1 gp120 expresses an array of high-mannose oligosaccharides ranging from Man5, Man6, to Man9 on these sites (76-78). These diverse oligomannose glycoforms of the 2G12 epitope on HIV-1 gp120 are likely to dilute any potential immune response to the epitope. This may partially explain why gp120 itself raises a limited number of 2G12-like antibodies. Further, carbohydrates themselves are generally poor immunogens, which may explain why 2G12-like neutralizing antibodies are rare in natural infection. Thus, it would be advantageous to provide a representative carbohydrate structure that would increase production of 2G12 neutralizing antibodies and that could be used as a component in a therapeutic composition.